Mr Trump himself would only learn of the document, which included an allegation strenuously denied by Mr Trump that he hired prostitutes to perform a lurid sex act in a Moscow hotel, in January 2017 when alerted by the FBI before taking office.

The US president has previously tweeted a claim that Britain’s intelligence services helped spy on his election campaign.

The seriousness with which the dossier was handled, being elevated to the country’s most senior intelligence figures, also suggests it was treated with gravity - in contrast to Mr Trump’s stance that the document is “fake” and “phony”.

Number 10 sources insist that Mrs May was never briefed on the Steele memos herself - indicating that intelligence chiefs decided to keep her at arm's length from the information, though why exactly remains unclear.

Mr Trump has singled out Mr Steele and his dossier in messages shared on Twitter more than 50 times since taking office, calling the former MI6 agent a “Trump hater” who was motivated by political bias.

However the full story of how aware British intelligence chiefs were of what their former agent had discovered can be revealed for the first time by the Telegraph.

Mr Steele’s so-called dossier was made up of 17 separate memos sent to Fusion GPS, a research firm based in Washington DC who contracted him and was being paid by lawyers for Hillary Clinton’s campaign.

Among them were that the Kremlin was trying to tilt the election in Mr Trump’s favour, that Trump campaign figures had secretly met Russians and that there was “kompromat” on Mr Trump himself in the form of a recording of a night he spent in a Moscow hotel in 2013.

Before the 2016 election, Mr Steele had met with FBI agents to alert them to his findings.

But when Mr Trump won an unexpected victory he decided it was time for Mrs May’s government to be informed.

They met at Farr’s house in Wimbledon, South London, a week after Mr Trump’s election victory and painstakingly went through the intelligence line by line, The Telegraph has learnt.

The pair met a second time, around 10 days later, to talk further and discuss new memos on the same subject which Mr Steele wrote after the election.

Farr died this February of cancer.

The outside of Orbis Business Intelligence, run by former British spy Christopher Steel.Credit:
Zuma Press/PA Images

Before being appointed head of the JIC in 2015, Farr had spent eight years as director-general at the Home Office’s Office for Security and Counter Terrorism, becoming close to Mrs May when she was Home Secretary.

So seriously was the material treated that Farr, a former MI6 officer, rapidly passed it up the chain of command, according to well-placed sources.

Andrew Parker, the MI5 director general, and Alex Younger, the MI6 chief, are both understood to have been briefed, according to the source.

The material was “marked up to the top”, according to sources. But Number 10 figures said categorically that Mrs May was never briefed on the dossier.

The reason for shielding the Prime Minister from such sensitive information remains unclear.

Possibilities include protecting her given the political need to develop a relationship with Mr Trump, uncertainty about elements of the dossier or a delay while verification took place.

Mr Trump was only told about the allegations on January 6 2017.

The then president-elect was briefed one-on-one by James Comey, then FBI director, about the dossier in Trump Tower, his Manhattan apartment block.

By the end of that month Mrs May had flown to Washington DC just days after Mr Trump’s inauguration, becoming the first world leader hosted in his White House. During the visit she offered Mr Trump a state visit on behalf of the Queen.